Sedimentary Environment And Dolomite Formation Mechanism Of Palaeogene Es~1 In Qikou Depression

Qikou Depression,located in Bohai Bay basin,where distributes a numerals outcrops of lacustrine carbonate rocks,has a great potential prospect of exploring for oil and gas.Qikou Depression has been showing giant oil-generating potential with the developmenting and producing of Beidagang Banqiao and Qijiawu regions.In the meanwhile,abounding oil and gas resources in dolomite of Es¹ were discovered in the process of well drilling for the west side of Qikou Depression.Therefore,the research for dolomite of Paleogene Es¹ in Qikou Depression is of great significance to tap the further potential for oil and gas.To discuss the rock type,sedimentary environment,diagenetic environment,and genesis of dolomite of Es¹,we have undertaken Sr isotope,C-O isotope major and trace element,rare earth element characteristics of the dolomite based on variety of technical methods,such as scanning electron microscope,cathode luminescence,and X-ray scattering techniques,as well as a detailed discussion of the transgression events during this period.The Es¹ in Qikou Depression consists of microcrystalline-micritic calcite dolomite,microcrystalline-micritic dolomite,and sparry bioclast dolomite with interbedded argillite.It has characteristics of rising water level,weakening hydrodynamic and enhancing reducibility during the time of Es¹ in the study area.On the whole,the rare earth element?REE?patterns of the dolomite of Es¹ are characterized with enrichment light REE and slightly depleted HREE;?Ce is relative to lacustrine dolomite;?La is in negative correlation with Y/Ho;REE is in positive correlation with Al,and negative correlation with Ca.Based on the evidences above,dolomite in study area are formed in lacustrine sedimentary environment.In addition,data points of ?¹³C and ?¹⁸O show banding distribution with the feature of increased simultaneously,indicating high correlation between ?¹³C and ?¹⁸O in dolomite.,reflecting an intensive closure of the Qikou Depression in this period.There are lots of characteristics of the study area as following: existing marine fossil such as stenothyra;Z,M,Sr/Ba,V/Ni,Th/U ratios of dolomite which indicate saline environments;87Sr/86 Sr values relative to seawater;?La??Y?Y/Ho values of most samples are out of the ranging values of lacustrine dolomite,on the contrary,representing strong seawater features.Based on the above evidences,there should be exist a transgression event during in Es¹ which lead to the dolomitization through bring out massive Mg²⁺ to lake basin,resulting in the salinization and raising PH of lake water,these factors.Dolomite show bright orange-red light;Carbon and oxygen isotopes indicate that the dolomite was formed in shallow burial environment without influence of fermentation;Degree of order and Mg/Ca values are quite low.We suggest the dolomite of Qikou and Koucun were formed by mode of seepage reflux base on the comprehensive analysis.Dolomite in this study show low light level and even no light;the feature of Carbon and oxygen isotopes indicate that the dolomite were formed in shallow burial environment and it was strongly influenced by fermentation;Degree of order is extremely low,but Mg/Ca values are higher than dolomite of Liujianfang and Qijiawu.Based on the above evidences,the dolomite of Liujianfang and Qijiawu was burial dolomite.